Electron discharge device of the velocity modulation type



Jan. 25, 1949; J. H. FREMLIN 2,459,805

ELECTRON DISCHARGE DEVICE OF THE VELOCITY MODULATION TYPE Filed Oct. 14, 1942 I [81 31:74 w mF/vr 5 I We I I 1354 184 cufimswr I DEV/CE llllllllllllllllllIIIIII IN VEN TOR. J'OH/V F/PL'MZ/N WMA A TTOF/VEY Patented Jan. 25, 1949 ELECTRON DISCHARGE DEVICE OF THE VELOCITY MODULATION TYPE John Heaven Fremlin, London, England, assignor,

by mesne assignments, to International Standan! Electric Corporation, New York, N. Y., a

corporation of Delaware Application October 14, 1942, Serial No. 462,029 In Great Britain December '12, 1941 The present invention relates to electron discharge devices of the kindin which a beam of electrons is directed across a first high frequency 3 Claims. (Cl. 3156) electric field to be velocity modulated thereby and then across a drift space and then across a second high frequency electric field which abstracts energy from the modulated beam. The two electric fields may be coupled together to produce oscillations in known manner.

In the design of such velocity-modulation oscillators for operation at short wavelengths with any considerable efliciency there are two requirements that are'difiicult to reconcile. Firstly. it is necessary that the electron beam after modulation by small oscillatory fields should be well bunched before crossing the energy-extracting field. This is the case because the losses in any particular circuit are proportional to the square of the amplitude of oscillation. The efiiciency of bunching'in a circuit of given resonant impedance at the point of application or the electron beam can be determined by measuring the startlng current (I51). or the current necessary for oscillation just to be maintained. If launchin is ineflicient this starting current will necessarily be high. In order, therefore, to reduce the effect of, losses it is necessary to make the bunching as efilcient as possible. 7

Secondly, it is necessary for the well formed bunch to cross as large :as possible a high frequency field in order that firect current energy may be efiiciently extracted at the output. This means that a large high frequency field must be obtained in the output circuit without over bunching of the electron stream by the modulating field. It will be observed that where the ratio of the amplitudes of the modulating'and energy extracting fields is fixed this condition clashes with the condition for low starting current described above, since with a large field and low starting electron current, over-modulatfon would take place. Thus oscillators which start easily would never give a very high efficiency. I

before specified provided with novel reflecting arrangements. I

According to the invention these arrangements comprise means. ior producing alon'gjhe portion of the path or the electron beamraiter passin the second high frequency electric field or energy abstracting field, a substantially constant potential positive with respect to the electron source of the beam and at the end of said portion of the beam path a .sudden drop in potential to substantially the potential of said electron source.

In this type of reflecting field, the faster electrons take the shorter time to reach the reflecting point and to return to the second high frequency field. The behaviour of an'electron in such a field may be likened to a ball thrown against a vertical wall and bouncing back. In this case, the faster the ball is thrown, the more rapidly will it return.

There is one other kind of reflecting field to which, however, the present invention does not relate, namely, a field in which the potential distribution gradually decreases from a positive potential at the second high frequency field to zero or slightly positive or negative potential with respect to the electron source of the beam at some distance from the said second high frequency field. In this case, the slower electrons take the shorter time to reach the reflecting point and return to the second high frequency field, and the behaviour of an electron may be likened to the behaviour of a ball thrown vertically into the air, a constant retardation occurring due to gravity, and the faster the ball is thrown up.

the longer will it take to return.

The invention will be better understood from the following description of one embodiment, given by Way of example and taken in conjunction with the accompanying drawings in which:

Figs. 1a and 11) show diagrammatically in plan and elevation an electron discharge device of the velocity-modulation type described in patent specification No. 2,320,860 issued'to John Heaver Fremlin on June 1, .1943.

Figs. 2 and 3 are graphs showing the relationship between the beam current and the efliciency of electron velocity modulated devices of known yp and Fig. 4 is a similar graph of a device accordint, to the invention. v

Referring to Figs. 1a and 11), one embodiment of the invention comprises .a coaxial line type resonator consisting of a length of a coaxial line formed by inner conductor 1 and outer con-1 ductor 2. An axial slot is provided through the inner conductor 1 and at diametrically opposite positions in the outer conductor, the slots in the two conductors being inalignment. to allow the passage of an electron beam therethrough. This beam is produced by a cathode 3 and controlling grid 4 situated opposite the slots on one side of the outer conductor. A reflecting electrode 5 is located at the diametrically opposite side of the outer conductor. A and B are adjusted by means of tips or flanges 6, 1, 8 and 9 as shown. The drift path for the electrons is across the inner conductor I.

In a coaxial line resonator of this type, which has been described in said patent specification No. 2,320,860, the beam is collected by'a collecting electrode in the position 5 and the modulation system satisfies'the second condition aforementioned, that is for large input powers 2. consider-' able efflciency, of the order of 35% may be obtained. On the other hand, the short drift path used makes the bunching by small'fields ineffi cient and the starting current is high. According to this invention, the collecting electrode is replaced by a reflecting electrode, so that the beam is reflected back across the gaps 'A and B and the transit time from the gap B to the reflecting electrode 5 and back again to the gap B is arranged to be an odd number of halfcycles of the high frequency field so that the reflected bunch of electrons in the beam will occur at the gap B alternately with the bunches of electrons in the incident beam. B this means the bunching is efiectively improved-and the output is increased by the energy extracted from the reflected beam. Oscillation will then be produced by very much smaller current than used without reflection. .-This is dueto the fact that an appreciable bunchin'g' of the beam is produced by very small oscillatory fields which cause correspondingly small losses. The reflection bunching mechanism comprises the flanges 9 which produce a constant potential along the portion of thebeam path passing between the flanges, the resonator being maintained at a positive potential with respect to cathode 3 and the sudden drop in potential is produced by the electrode 5 which is maintained substantially at cathode potential and maybe slightly. positive zero or negative in potential with respect to cathode. As the amplitude of oscillation increases, the output due to this reflected beam, of course, likewise increases at first and then begins to fall off as the incident beam is .overbunched. In this condition, however, the amplitude of the oscillation will be sufilcient to cause'considerable bunching of the incident electron stream during its first passage through the drift tube and the energy transfer on the first transit will be appreciable. j

The operation of the arrangements according to 'the invention may perhaps be more easily understood by reference to Figs. 2, 3 and 4; Fig. 2 represents the variation with beam current of the efficiency of a coaxial line type device in which the electron beam is collected after a single transit, the starting current, In, being considerable, and the efliciency increasing towardsalimiting value e1. g

Fig. 3 represents the efiiciency for the energy extraction from the beam after reflection showing the much smaller starting current IsZ (this may easily be made one twentieth of I51 in 'a'practical system) and the ultimate efliciency e2. Fig. 4 shows the efliciency curve actually obtained in practice from the embodiment of the invention described hereinbefore showing both thelow starting current of Fig. 3 and the high flnal efficiency of'Fig. 2. I

The Width of the gaps to pass the said apertures. Each aperture may be in ,anindividual; chamber in which case, the chambers are coupled to produce and maintain the electric high frequency oscillations as is well known to those well versed in the art.

r VVh'at is claimed is:

1, An' .ultra high frequency apparatus comprising an electron discharge device of the velocity modulation. type including an electron gun for producing and directing a beam of electrons along a beam path, said gun including a cathode, and pliu'ality of means mounted in the order named along said beam path from said gun, means for producing an ultrahigh frequency electricfield for velocity modulating electrons injsaidbeam, means providing a first drift space, means for extracting ultra high frequency energy from said velocity modulated beam and an electrode spaced along said beam from said energy extracting mea'ns for reflecting'the electrons in said beam back throu h said extractin means, the space between said' extractingme'ans and, said reflecting electrode. forminga second driftspa'ce, said ultra high ,fiidquency apparatus further. comprising means connected to said energy "extracting means for applying 'a potential thereto and means con-j ne'ctedtosaid cathode and said reflecting electrode for applying ac'ommon potential thereto, said last-mentioned {potential being negative with respect to said firstmientioned potential and providing an electron transit-time from said; energy extracting means :to said reflecting electrode and return corresponding to an odd number of half cycles of the. high frequency field whereby. the. rd' s. a bu h h e the arrive th energy'extracting means and arrive atsaidene sy extracting means cluring the intervals'between the arrival of bunches in the incident beam,

2. An oscillation' gen erator apparatusof highl efficiency c omprisi'ng ,ar' electron discharge "dcvice" of the velocity'modulation type including an electron gun-tori producing and directing a beam of electrons along ajbleam path, said g'un'inc'ludg. ing a cathode, and a; plurality. zoi 'meansinouned in theorder named along said b eam path from said gun, meansfor producing anu1trahigh ire quencyielect'ric. field for velocity modulating elec-j trons in said beam, means providing a. first drift space, means for extracting ultra high frequency energy from said velocity modulated beam, means providing a second drift space and anelectrode mounted at a distance along said beam'pathirom said'extracting means for reflecting the electrons in said beam backthrough said extracting means, saidoscillation generator further comprising means connected to said cathode and said re fleeting electrode for applying a common potential thereto and .means connected-to said means providing said second drift space for applying, thereto a potential positive with respect to that, of said cathode, said potentials providing an electrontime .irom said; energy extracting means, to salctweflecting electrode and return corres ond:

ing to an odd number of half cycles of the high 7 frequency field whereby the electrons are bunched when they arrive at the energy extracting means and arrive at said energy extracting means during the intervals between the arrival of bunches in the incident beam. 7

3. An oscillation generator apparatus of high eificiency according to claim 2, wherein said means for producing an ultra high frequency electric field, said means providing a first drift space, and said extracting means comprise a coaxial line type resonator with diametrically arranged slots therein aligned with said beam path, said beam path extending through first and second gaps between the inner and outer conductors of said resonator and wherein said means providing a second drift space comprise flanges at the slot in the outer conductor adjacent said second gap, said flanges being parallel to said beam path.

JOHN HEAVER FREMLIN! 6 REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PA'IENTS Number Name Date 7 2,093,151 Linder Sept. 14, 1937 2,170,219 Seller Aug. 22, 1939 I 2,190,511 Cage Feb. 13, 1940 2,190,515 Hahn Febrl3, 1940 2,216,169 George et al Oct. 1, .1940

2,242,275 Varian May 20, 1941 7 2,250,511 Varian et al July 29, 1941 2,259,690 Hansen et a1. Oct. 21, 1941 2,278,210 Morton Mar. 31, 1942 2,289,846 Litton July 14, 1942 2,338,306 Smyth Jan. 4, 1944 2,383,343 Ryan Aug. 21, 1945 FOREIGN PATENTS Number Country Date Great Britain June 25, 1941 

